U.S. public health experts warn that global warming and air pollution will worsen the epidemic of asthma among poor children in cities unless steps are taken to reduce fossil fuels burned by cars, trucks, and buses.

The World Health Organization (WHO) says global rates of asthma are rising an average of 50 percent a decade, now affecting 100 million to 150 million people and killing 180,000 people a year.

A new report from the Harvard Medical School and the American Public Health Association said that the problem will worsen as levels of carbon dioxide rise in the atmosphere. The compound is believed to contribute to rising temperatures by trapping heat, like a greenhouse, that would otherwise escape the planet, but the director of Harvard's Center for Health and the Global Environment, Dr. Paul Epstein, said that carbon dioxide also promotes plant pollen production, soil bacteria, fungi and favors growth of certain weeds like ragweed and poison ivy. "This is new, so there is a lot we don't know and a lot to study, but it's important to say that we are seeing some very troubling new evidence that may be exacerbating the allergies and the assaults on our respiratory systems, particularly those of children," he said.

The report said that other emissions from motor vehicles form a smog that can cause and worsen asthma, while diesel particles help deliver pollen and molds deep into the sacs of the lungs.

The American Public Health Association's executive director, Dr. Georges Benjamin, told a Washington news briefing that poor city dwellers are affected most because of urban transportation patterns that concentrate pollution. He said that city children are the hardest hit.

"Children are not little adults," he said. "They breathe faster, they pull in more air, so if the air is dirty, they breathe much more soot and ozone than a comparable adult. Also, their bodies are growing and changing, so they are much more vulnerable to these environmental hazards."

The public health experts suggest fossil fuels such as gasoline cause air pollution and climate change, are hurting air quality in cities worldwide. They argue that the impact of pollution can be compounded by extreme weather events, whose intensity and frequency are increasing as the climate changes with increasing amounts of carbon dioxide.

Their report notes that atmospheric concentrations of the chemical are at high levels the Earth has not experienced for at least 420,000 years.

Christine Rogers is a senior researcher at the Harvard School of Public Health in Boston.

"This is a real wake up call for people who mistakenly think that global warming is going to be a problem only far off in the future," Mrs. Rogers said. "The problem is here today for these children and it appears it is only going to get worse."

The Harvard and American Public Health Association report urges strategies that improve air quality and enhance the livability of urban centers. These include converting from fossil fuel use to alternative energy sources, hybrid motor vehicles that can burn alternative fuels as well as gasoline and improved public transportation.

The experts argue that transforming to clean energy use would produce many new industries and jobs and boost international trade.